1. Technical Field of the Invention
The present invention relates to a method for fabricating optical fibers and, in particular, to a method for fabricating optical fibers calling for insertion of core materials into a refractory cladding tube such as SiO.sub.2.
2. Description of the Prior Art
It has long been of interest in the art to fabricate optical fibers having cores which contain various rare earth ions because certain rare earth ions are known to lase in glass or to give other desirable properties such as faraday rotation. For example, ions such as Nd.sup.3+, Yb.sup.3+, Er.sup.3+, Pr.sup.3+, Tm.sup.3+ and Ho.sup.3+ have been made to lase in glass. Of these, trivalent neodymium has perhaps been the most important because it has been made to lase at room temperature with high efficiency. Further, with the advent of low-loss fibers made predominantly from fused silica, it became desirable to develop fiber lasers in which the composition of the rare earth doped core was compatible with a fused silica cladding. This would ensure that the laser fibers would be compatible in numerical aperture (NA) and in other respects with "communications grade" low-loss fused silica single mode or multimode fibers.
A number of prior art methods have been tried to produce such low-loss, rare earth doped, fused silica fibers having cores with high rare earth content. For example, one method used for fabricating low-loss, rare earth doped, fused silica fibers is similar in some respects to the methods used today for making low-loss "communications grade", fused silica optical fibers. In this particular method, a modification of the Modified Chemical Vapor Deposition (MCVD) method, which is commonly referred to as inside chemical vapor deposition, rare earth ions are introduced into the core of the fiber by admitting volatile compounds of the desired rare earth ions into the reaction zone within a preform. A chemical reaction occurs in the reaction zone, and another compound, which contains the rare earth ion, is deposited on the inside the preform as a continuous layer of glass or as a soot. The deposited material is then ion solidated into a continuous layer of glass, if necessary, and is then incorporated into the core of the optical fiber when the preform is subsequently collapsed and drawn down in to the optical fiber. This method suffers in one respect due to the difficulty in finding volatile and thermally stable materials formed with rare earth ions or other ions which it may be desirable to add to the glass to adjust its properties. The volatility is important in providing the compound in the vapor phase at reasonably low temperatures, and the thermal stability is important in transporting the compound to the reaction zone in the apparatus. Further, mass flow controllers and appropriate carrier gases are needed to deliver the compounds on a controlled basis through transport lines to ensure that the compounds remain in the gaseous state until they reach the reaction zone.
In another prior art method, referred to as a "rod-in-tube" method, various compatible combination of core and cladding glasses are used which are characterized by the fact that they do not change on drawing the fiber, and, further, the final fiber is strong, light guiding and with all the other desirable properties generally ascribed to good fibers. These can be so called hard glasses, i.e., mostly SiO.sub.2, or soft glasses, i.e., alkali, alkaline earth silicates. In fact, the first rare earth doped laser fibers made consisted of soft glasses for both core and cladding. These soft glasses as, for example, window glass are typically alkali and alkali earth silicates that are commonly and commercially available in a wide range of compositions. In this prior art method, a rod of soft glass containing a rare earth compound is placed inside a tube of an appropriate soft glass. The combination is then drawn down into a fiber whose core contains the rare earth ion.
When the above-described method is applied to placing a soft glass rod into a tube of fused silica and attempts made to draw down the combination into a fiber, this method fails either because stresses are formed or because bubbles are formed by the volatile constituents in the core which "blow out" the side of the tube. This is because the coefficient of expansion for a typical soft glass is of the order of 90.times.10.sup.-7 /.degree. C. whereas the expansion coefficient of fused silica is roughly 3-5.times.10.sup.-7 /.degree. C. This radical difference in coefficient of expansion causes optical fibers fabricated with a soft glass core and a fused silica cladding to fracture on cooling, the fracture extending outward from the corecladding interface. Further, polishing the ends of the fiber does not remove the fractures as they will be generated to extend from the polished end.
As a result of the above, there is a need in the art for a method for fabricating optical fibers and, in particular, for fabricating optical fibers having rare earth doped, fused silica cores surrounded by a fused silica cladding which is simpler and less expensive than the MCVD method and which provides a simple method like that used to fabricate optical fibers having a soft glass core embedded in a soft glass cladding.